Piece of cake. As you progress through the game, you'll bash pretty much anything, that will then give you "Helmuts" - these are used as money at the upgrade booth, located in the first center area right next to the mini-game area. This is the ONLY upgrade area (booth) there is in the game.
One trick is to keep exiting this area and going back to the "front" starting point in the game. As you do this, all of the smashable wood wagons, etc. are reset, and you can bash them again & again, by just re-entering each of these two areas if you want to get these upgrades first before proceeding. I actually recommend doing this, as the combos available are excellent and will do you good as you then progress through the rest of the game.
The only upgrade that is NOT doable from the start (unless you feel like bashing stuff for hours) is the $5000 Tornado whirlwind. Just come back when you get 5 grand, it's not really needed until later.
The rest of the upgrades are cheap, and should be gotten as soon as possible to make the game more fun.
This cheev will pop when you buy the few things that are available there.